Method and apparatus for rapidly reconfiguring computers networks executing the spanning tree algorithm

ABSTRACT

A method that rapidly reconfigures a computer network having a plurality of devices executing the spanning tree algorithm. First, one or more devices are configured and arranged so that one port, providing connectivity to the root, is in the forwarding state and the remaining ports, providing connectivity to the root, are in the blocked state. Next, one or more of the blocked ports are designated as back-up ports. Upon detection of a failure at the active forwarding port, one of the back-up ports immediately transitions from blocked to forwarding, thereby becoming the new active port for the device. Following the transition to a new active port, dummy multicast messages are transmitted, each containing the source address of an entity directly coupled to the affected device or downstream thereof. By examining the dummy multicast messages, other devices in the network learn to use to the new forwarding port of the affected device. Rapid reconfiguration of the network is also provided upon detection of a new or repaired link or device representing a better path toward the root. The method is also compatible with networks supporting virtual local area network (VLAN) designations and allows load balancing among different VLANs.

FIELD OF THE INVENTION

[0001] The present invention relates generally to computer networks, andmore specifically, to a method and apparatus for rapidly reconfiguring acomputer network following a network change.

BACKGROUND OF THE INVENTION

[0002] A computer network typically comprises a plurality ofinterconnected entities. An entity may consist of any device, such as acomputer or end station, that “sources” (i.e., transmits) or “sinks”(i.e., receives) data frames. A common type of computer network is alocal area network (“LAN”) which typically refers to a privately ownednetwork within a single building or campus. LANs typically employ a datacommunication protocol (LAN standard), such as Ethernet, FDDI or tokenring, that defines the functions performed by data link and physicallayers of a communications architecture (i.e., a protocol stack). Inmany instances, several LANs may be interconnected by point-to-pointlinks, microwave transceivers, satellite hook-ups, etc. to form a widearea network (“WAN”) or internet that may span an entire country orcontinent.

[0003] One or more intermediate devices are often used to couple LANstogether and allow the corresponding entities to exchange information.For example, a switch may be utilized to provide a “switching” functionfor transferring information, such as data frames, among entities of acomputer network. Typically, the switch is a computer and includes aplurality of ports that couple the switch to the other entities. Portsused to couple switches to each other are generally referred to as atrunk ports, whereas ports used to couple a switch to LANs or endstations are generally referred to as local ports. The switchingfunction includes receiving data at a source port from an entity andtransferring that data to at least one destination port for receipt byanother entity.

[0004] Switches typically learn which destination port to use in orderto reach a particular entity by noting on which source port the lastmessage originating from that entity was received. This information isthen stored by each switch in a block of memory referred to as afiltering database. Thereafter, when a message addressed to a givenentity is received on a source port, the switch looks up the entity inits filtering database and identifies the appropriate destination portto utilize in order to reach that entity. If no destination port isidentified in the filtering database, the switch floods the message outall ports, except the port on which the message was received. Messagesaddressed to broadcast or multicast addresses are also flooded.

[0005] To prevent the information in the filtering database frombecoming stale, each entry is “aged out” by a corresponding timer.Specifically, when an entry is first added to the filtering database,the respective timer is activated. Thereafter, each time the switchreceives a subsequent message from this entity on the same source port,it simply resets the timer. Pursuant to standards set forth by theInstitute of Electrical and Electronics Engineers (IEEE), the defaultvalue of the timer is five minutes. See IEEE Standard 802.1D. Thus,provided the switch receives a message from a particular entity at leastevery five minutes, the timer will keep being reset and thecorresponding entry will not be discarded. If the switch stops receivingmessages, the timer will expire and the corresponding entry will bediscarded. Once the entry ages out, any messages subsequently receivedfor this entity must be flooded, until the switch receives anothermessage from the entity and thereby learns the correct destination port.

[0006] Additionally, most computer networks include redundantcommunications paths so that a failure of any given link does notisolate any portion of the network. Such networks are typically referredto as meshed or partially meshed networks. The existence of redundantlinks, however, may cause the formation of circuitous paths or “loops”within the network. Loops are highly undesirable because data frames maytraverse the loops indefinitely. Furthermore, as described above, manydevices such as switches or bridges replicate (i.e., flood) frames whosedestination port is not known or which are directed to broadcast ormulticast addresses, resulting in a proliferation of data frames alongloops. The resulting traffic effectively overwhelms the network.

[0007] Spanning Tree Algorithm

[0008] To avoid the formation of loops, devices, such as switches orbridges, execute a spanning tree algorithm. This algorithm effectively“severs” the redundant links within the network. Specifically, switchesexchange special messages called bridge protocol data unit (BPDU) framesthat allow them to calculate a spanning tree or active topology, whichis a subset of the network that is loop-free (i.e., a tree) and yetconnects every pair of LANs within the network (i.e., the tree isspanning). Using information contained in the BPDU frames, the switchescalculate the tree in accordance with the algorithm and typically electto sever or block all of the redundant links, leaving a singlecommunications path.

[0009] In particular, execution of the spanning tree algorithm causesthe switches to elect a single switch, among all the switches withineach network, to be the “root” switch. Each switch has a uniquenumerical identifier (switch ID) and the root is the switch having thelowest switch ID numeric value. In addition, for each LAN coupled tomore than one switch, a single “designated switch” is elected that willforward frames from the LAN toward the root. The designated switch istypically the one closest to the root. By establishing designatedswitches, connectivity to all LANs, where physically possible, isassured.

[0010] Each switch within the network also selects one port, known asits “root port” which gives the lowest cost path (e.g., the fewestnumber of hops, assuming all links have the same cost) from the switchto the root. The root ports and designated switch ports are selected forinclusion in the spanning tree and are placed in a forwarding state sothat data frames may be forwarded to and from these ports and thus ontothe corresponding paths or links. Ports not included within the spanningare placed in a blocked state. When a port is in the blocked state, dataframes will not be forwarded to or received from the port. At the root,all ports are designated ports and are therefore placed in theforwarding state, except for some self-looping ports, if any. Aself-looping port is a port coupled to another port at the same switch.

[0011] Each BPDU typically includes, in part, the following information:the identifier of the switch assumed to be the root (by the switchtransmitting the BPDU), the root path cost to the assumed root and theidentifier of the switch transmitting the BPDU. Upon receipt of a BPDU,its contents are examined and compared with similar information (i.e.,assumed root ID, lowest root path cost and switch ID) stored by thereceiving switch. If the information from the received BPDU is “better”than the stored information, the switch adopts the better informationand begins transmitting it (adding the cost associated with thereceiving port to the root path cost) through its ports, except for theport on which the “better” information was received. Eventually, allswitches will agree on the root and each will be able to identify whichof its ports presents the lowest cost path to the root (i.e., its rootport).

[0012] Depending on the configuration of a given network, the locationof the root can significantly affect the distance that messages musttravel. For example, many networks include a plurality of switchesdesignated as access switches that provide connectivity to LANs, endstations, etc., and a plurality of backbone switches that, in turn,interconnect the various access switches. If the root is located at anaccess switch and the principal server utilized by the end stations(i.e., clients) is coupled to a backbone switch, the average distancebetween end stations and the primary server may be quite high, resultingin inefficient network operation. In addition, the backbone switches maybecome partitioned as ports between them are blocked. To reduce theaverage distance and avoid partitioning of the backbone switches, it isdesirable to locate the root at a backbone switch. Switch IDs, moreover,include a fixed portion and a settable portion. By substantiallydecreasing the value of the settable portion of the identifier for aselected switch, a network administrator may “force” the network tochoose the selected switch as the root.

[0013] To identify which switch should be the designated switch,switches again compare information in received BPDUs with their storedinformation. If the root path cost stored by a first switch is lowerthan the root path cost contained in BPDUs received from a secondswitch, then the first switch is the designated switch. If the root pathcost for both the first and second switches is the same, the firstswitch compares the next informational element in the BPDU, i.e., theswitch IDs. If the switch ID of the first switch is less than the ID ofthe second switch, then the first switch is the designated switch,otherwise the second switch is the designated switch.

[0014] In accordance with the spanning tree algorithm, the root switchgenerates and transmits BPDUs from its ports every hello time which is asettable parameter. Pursuant to IEEE standards, the default hello timeis two seconds. In response to receiving BPDUs, switches transmit theirown BPDUs. Thus every two seconds BPDUs are propagated through thenetwork. BPDU information, moreover, like entity address information, issubject to being aged out and discarded. Typically, a timer isassociated with the BPDU information stored for each port of a switch.The timer is set to a value referred to as the maximum age which isloaded into BPDUs generated by the root switch and copied by the otherswitches. An example of a default maximum age value is twenty seconds.As BPDUs are received, their contents are examined. If the contentsmatch the information already stored for that port, the timer is reset.Accordingly, by receiving consistent BPDUs every hello time, which issignificantly less than the maximum age, the current BPDU information ismaintained and the accuracy of the spanning tree or active topology isconfirmed.

[0015] If a switch stops receiving BPDUs on its root port, indicating apossible link or device failure, the corresponding timer will expire andthe information will be discarded. In response, the switch will select anew root port based upon the next best information it has, and begintransmitting BPDUs through its other ports. Similarly, as links ordevices are repaired or added, a switch may receive BPDUs containingbetter information than that stored for a particular port, therebycausing the switch to replace the previously stored information, asdescribed above.

[0016] As BPDU information is up-dated and/or timed-out, the spanningtree is recalculated and ports may transition from the blocked state tothe forwarding state and vice versa. That is, as a result of new BPDUinformation, a previously blocked port may learn that it is now the rootport or the designated port for a given LAN. Rather than transitiondirectly from the blocked state to the forwarding state, portstransition through two intermediate states: a listening state and alearning state. In the listening state, a port waits for informationindicating that it should return to the blocked state. If, by the end ofa preset time, no such information is received, the port transitions tothe learning state. In the learning state, a port still blocks thereceiving and forwarding of frames, but received frames are examined andthe corresponding location information is stored, as described above. Atthe end of a second preset time, the port transitions from the learningstate to the forwarding state, thereby allowing frames to be forwardedand received at the port. The time spent in each of the listening andthe learning states is referred to as the forwarding delay.

[0017] As ports transition between the blocked and forwarding states,entities may appear to move from one port to another. To preventswitches from distributing messages based upon incorrect information,switches quickly age-out and discard the “old” information in theirfiltering databases. More specifically, upon detection of a change inthe spanning tree, switches transmit Topology Change NotificationProtocol Data Unit (TCN-PDU) frames toward the root. The format of theTCN-PDU frame is well known (see IEEE 802.1D standard) and, thus, willnot be described herein. The TCN-PDU is propagated hop-by-hop until itreaches the root which confirms receipt of the TCN-PDU by setting atopology change flag in all BPDUs subsequently transmitted by the rootfor a period of time. Other switches, receiving these BPDUs, note thatthe topology change flag has been set, thereby alerting them to thechange in the active topology. In response, switches significantly lowerthe aging time associated with their filtering databases which, asdescribed above, contain destination information corresponding to theentities within the network. Specifically, switches replace the defaultaging time of five minutes with the forwarding delay time, which isgenerally fifteen seconds according to the IEEE standards. Informationcontained in the filtering databases is thus quickly discarded.

[0018] Although the spanning tree algorithm is able to maintain aloop-free tree despite network changes, recalculation of the spanningtree is a time consuming process. For example, as described above, themaximum age of BPDUs (i.e., the length of time that BPDU information iskept) is typically twenty seconds and the forwarding delay time (i.e.,the length of time that ports are to remain in each of the listening andlearning states) is fifteen seconds. As a result, recalculation of thespanning tree following a network change takes approximately fiftyseconds (e.g., twenty seconds for BPDU information to time out, fifteenseconds in the listening state and another fifteen seconds in thelearning state).

[0019] During this recalculation period, message delivery is oftendelayed as ports transition between states. That is, ports in thelistening and learning states do not forward or receive messages. To thenetwork users, these delays are perceived as service interruptions,which may present significant problems, especially on high-reliablenetworks. In addition, certain applications, protocols or processes maytime-out and shut down during the reconfiguration process, resulting ineven greater disruption to the system. Another disadvantage relates tosubsequent message distribution. Following the reconfiguration process,messages are flooded across the network until the “new” destinationports are learned and the aging time returned to five minutes. Suchflooding of messages often consumes substantial communications andprocessor resources.

SUMMARY OF THE INVENTION

[0020] It is an object of the present invention to provide a method andapparatus for reducing the time necessary to reconfigure the networkfollowing a change, such as a link failure or recovery.

[0021] It is a further object of the present invention to provide amethod and apparatus for defining a series of back-up ports which mayimmediately begin forwarding data messages following a failure at anactive port.

[0022] It is another object of the present invention to provide a methodand apparatus for defining primary and back-up root devices such thatthe back-up becomes the new root upon failure of the primary.

[0023] Another object of the present invention is to provide a methodand apparatus for balancing message traffic across several links of acomputer network.

[0024] Yet another object of the present invention is to provide amethod and apparatus that is compatible with non-enabled devices.

[0025] Briefly, the invention relates to a method and apparatus forrapidly reconfiguring a computer network. The network preferablyincludes a plurality of devices executing the spanning tree algorithm soas to elect a root and place the ports of the devices in either aforwarding or blocked state. In accordance with the method, one or moredevices are configured and arranged so that one trunk port is in theforwarding state and other trunk ports are in the blocked state.Additionally, one or more of the blocked ports are designated as back-upports. Upon detection of a failure at the active forwarding port, thestate of one of the back-up ports immediately transitions from blockedto forwarding, thereby becoming the new active port for the device.Advantageously, the selected back-up port does not transition throughany intermediary states (such as the listening or learning states) inmoving from blocked to forwarding. Accordingly, the time required totransition to a new active port capable of forwarding data messages issubstantially reduced.

[0026] Upon transition to the new forwarding port, the device beginstransmitting “dummy” multicast messages through the new port. Thesedummy multicast messages carry the source address of each entity that isdirectly coupled to the device with the new active port or downstreamthereof (relative to the root) and are received by other devices in thecomputer network. Upon receipt, the other devices examine the contentsof these messages and note the port on which they were received, whichmay differ from the port on which messages from these entities werepreviously received (i.e., before the failure and subsequent replacementof the device's active port). It is through this process that otherdevices within the network learn to utilize the new forwarding port,rather than the failed port, when directing messages to these entities.Notably, the transition to a new forwarding port is accomplished withoutother devices having to discard the contents of their filteringdatabases and, thus, the flooding of messages following a network changeis substantially reduced.

[0027] In the illustrated embodiment, the method and apparatusmanifests, in part, as a series of novel commands that may be entered atthe devices. The devices, moreover, may be classified as either accessswitches or backbone switches. Access switches are preferably coupled toentities (e.g., LANs, end stations, etc.) whereas backbone switchesprovide the interconnections between access switches. A first command,Become_Root_Primary, is preferably entered at a first backbone switchand significantly lowers the value of the first backbone switch'snumeric ID, thereby forcing it to become the root upon execution of thespanning tree algorithm. This command also modifies certain parametersassociated with the spanning tree algorithm to further reducereconfiguration time. A second command, Become_Root_Secondary,preferably entered at a second backbone switch, adjusts the secondbackbone switch's ID to a value between a default value and the valuespecified in the Become_Root_Primary command. The Become_Root_Secondarycommand thus causes the second backbone switch to become the new rootupon a failure of the first backbone switch.

[0028] A third command, Enable_Uplinkfast, is preferably entered at eachaccess switch. This command substantially increases the values of accessswitches' IDs, effectively precluding any access switch from becomingthe root. This command also increases the path costs associated witheach port of the access switches. By raising the path costs, accessswitches are less likely to become designated switches. As a result,only one trunk port (i.e., the root port) for each access switch isgenerally placed in the forwarding state. The remaining trunk portswhich normally connect the access switch to the corresponding backboneswitches are blocked.

[0029] The Enable_Uplinkfast command also designates the blocked trunkports of the corresponding access switch, except self-looping ports, aspossible back-up root ports. Upon failure of the current root port, thiscommand additionally configures the access switch to immediatelytransition one of its blocked trunk ports to the forwarding state and toalso begin transmitting dummy multicast messages through the new port,as mentioned above. Upon detection of a new or repaired link or devicerepresenting a better path toward the root, this command additionallyconfigures the access switch to transition to the new path withoutsuffering a loss of connectivity. Reconfiguration of the network maythus be accomplished substantially sooner than the time required by theconventional spanning tree algorithm while still avoiding the formationof loops.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The above and further advantages of the invention may be betterunderstood by referring to the following description in conjunction withthe accompanying drawings, in which:

[0031]FIG. 1 is a highly schematic diagram of a computer network inaccordance with the present invention;

[0032]FIG. 2 is a partial block diagram of a device in accordance with apreferred embodiment of the present invention;

[0033] FIGS. 3A-E are flow diagrams of methods used to rapidlyreconfigure the computer network; and

[0034]FIG. 4 is a block diagram of a dummy multicast message inaccordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0035]FIG. 1 illustrates a partially meshed computer network 100 inaccordance with the present invention. The network 100 preferablycomprises a plurality of local area networks (LANs) 102-109 each ofwhich is coupled to one or more access switches 114-116. Access switches114-116 are interconnected with each other through a plurality ofbackbone switches 120-125. Specifically, access switches 114-116 aregenerally connected to the backbone switches 120-125 through a series oflinks 128, such as point-to-point links, although they may be connectedvia a shared media (e.g., LAN 109). The backbone switches 120-125 aresimilarly interconnected by links 128. Each access switch 114-116 andbackbone switch 120-125, moreover, includes a plurality of ports which,if coupled to a LAN 102-109, are referred to as local ports 118 and ifcoupled to links 128 are referred to as trunk ports 119.

[0036] Network 100 further includes a plurality of servers 112 a-112 c,such as file servers, print servers, etc., each coupled to the localport 118 of one or more access or backbone switches. Each LAN 102-109includes one or more hosts or end stations (not shown) that may sourceor sink data frames to one another or to the servers 112 a-112 c overthe network 100. One or more routers 130 and 131 may also be provided toadd functionality to network 100. Preferably, each router 130, 131 iscoupled to a backbone switch, such as backbone switches 120 and 121, bya corresponding link 128.

[0037] Links 128 represent communications paths between variouscomponents of the network 100, that carry messages, such as data frames,between switches 114-116, 120-125 and routers 130, 131. In addition,each switch and router, such as access switch 114, identifies its localand trunk ports 118 and 119 by numbers (e.g., port number one, portnumber two, port number three, etc.) Switches 114-116, 120-125 androuters 130, 131 are thus able to associate specific ports with theLANs, switches, routers, servers, etc. that are coupled thereto orotherwise accessible through a given port.

[0038] It should be understood that the network 100 of FIG. 1 is meantfor illustrative purposes only and that the present invention willoperate with other network designs having possibly far more complextopologies. For example, one or more distribution switches may beinterleaved between the backbone switches and the access switches, twoaccess switches may be directly connected, etc. It will also beunderstood to those skilled in the art that there is no distinction fromthe spanning tree point of view between local and trunk ports orpoint-to-point links and shared links (e.g., LANs). The distinctionbetween local and trunk ports is made for purposes of explanation only.

[0039] As shown, network 100 includes redundant communication pathsinterconnecting the access and backbone switches 114-116 and 120-125.The existence of such redundant links prevents portions of the network100 from becoming isolated should any constituent link or device fail.Such redundancy, however, also results in the creation of loops, which,as described above, are highly undesirable. To avoid the creation ofloops, these switches execute the spanning tree algorithm, as describedherein.

[0040]FIG. 2 is a partial block diagram of a switch 214 in accordancewith the present invention which corresponds to access switch 114 ofFIG. 1. A switch suitable for use with the present invention isdescribed in commonly owned U.S. patent application Ser. No. 08/623,142filed Mar. 28, 1996 and entitled INTERSWITCH LINK MECHANISM FORCONNECTING HIGH-PERFORMANCE NETWORK SWITCHES which is herebyincorporated by reference in its entirety. Switch 214 includes aplurality of ports 230 each of which is preferably identified by anumber (e.g., port numbers one through eight). Frame transmission andreception objects (not shown) may be associated with each port 230 suchthat frames received at a given port 230 may be captured, and frames tobe transmitted by switch 214 may be delivered to a given port. Framereception and transmission objects are preferably message storagestructures, such as queues.

[0041] Switch 214 further includes a protocol entity 232. The protocolentity 232 preferably includes a rapid reconfiguration entity 234, atleast one spanning tree state machine engine 236 and a forwarding entity238. The rapid reconfiguration entity 234 is coupled to areconfiguration memory 240. Memory 240 includes a plurality of cells 240a-240 e for storing the unique numeric switch identifier (ID)corresponding to switch 214, the assigned path cost corresponding toeach port 230 and the values corresponding to the hello time, maximumage and forward delay parameters of the spanning tree algorithm. As setforth above, a switch ID includes a fixed portion and a settableportion.

[0042] Each spanning tree engine 236 is similarly coupled to a spanningtree table 242 having a plurality of rows and columns. Each row or entry243 of table 242 is preferably associated with a port 230 of the switch214. The columns or cells, moreover, include a root ID cell 242 a, aroot path cost cell 242 b and a corresponding timer cell 242 c. Itshould be understood that table 242 may include additional cells.

[0043] The forwarding entity 238 is coupled to a filtering database 244configured to store address information corresponding to the entities ofnetwork 100. Specifically, filtering database 244 has a plurality ofcolumns or cells, including a destination address cell, a destinationport cell and a corresponding timer cell. Each row or entry in thefiltering database 244 preferably corresponds to a particular entity.

[0044] It will be understood to those skilled in the art that memory 240and tables 242 and 244 may be implemented as content addressable memory(CAM) devices and that entities 234 and 238 and state machine engine 236may comprise registers and combinational logic configured and arrangedto produce sequential logic circuits. In the illustrated embodiment,entities 234 and 238 and state machine engine 236 preferably compriseprogrammable processing elements containing software programs pertainingto the methods described herein and executable by the processingelements. Other computer readable media may also be used to store andexecute the program instructions.

[0045] The method and apparatus of the present invention is preferablyimplemented, in part, through the operation of a plurality of novelcommands entered at the various switches of the network 100 (FIG. 1),typically by a user or network administrator.

[0046]FIG. 3A is a flow diagram of a Become_Root_Primary command 300that is preferably entered at a first backbone switch (e.g., backboneswitch 120). Selection of the particular backbone switch as the primaryroot may depend on where the most utilized server, such as server 112 b,is located. That is, to reduce the average distance that messages musttravel, the Become_Root_Primary command 300 is preferably entered at thebackbone switch coupled to the most heavily utilized server, as shown atblock 310. In response to the Become_Root_Primary command 300, the rapidreconfiguration entity 234 of the selected backbone switch 120, at block312, modifies the settable portion of the corresponding switch ID storedat reconfiguration memory 240. In particular, the value of the switch IDis significantly lowered to ensure that the selected backbone switch 120will be elected the root switch upon execution of the spanning treealgorithm. For example, the switch ID may be reduced from the defaultvalue of “32768” specified by the IEEE standards to “8192”. In addition,the Become_Root_Primary command, at block 314, also modifies the hellotime, maximum age and forward delay parameters of the spanning treealgorithm as a function of network diameter as set forth in Table 1.Network diameter is defined by the IEEE 802.1D standard as the maximumnumber of devices between any two points of attachment of end stationsand is an argument of the Become_Root_Primary command 300. TABLE 1Network Diameter 2 3 4 5 6 7 hello time 1.0 1.0 1.0  1.0  1.0  1.0maximum age 6.0 7.0 8.5 10.0 11.5 13.0 forward delay 4.0 4.8 5.8  6.8 7.8  8.8

[0047] These values are then stored at memory 240 by reconfigurationentity 234. To implement the Become_Root_Primary command 300, entity 234may maintain the information of Table 1 in memory, such as memory 240.

[0048] In case the first backbone switch 120 should ever fail, a secondcommand is preferably entered to establish a back-up root. Referring tothe flow diagram of FIG. 3B, a Become_Root_Secondary command 320 ispreferably entered at a second backbone switch (e.g., backbone switch121), as shown at block 322. The Become_Root_Secondary command 320similarly modifies the settable portion of the switch ID of the secondbackbone switch 121 that is stored at the respective reconfigurationmemory 240, as shown by block 324. Specifically, the value of the switchID is modified so as to be between the default IEEE 802.1D value (i.e.,“32768”) and the value specified by the Become_Root_Primary command 300(i.e., “8192”). For example, the switch ID may be set to “16384”. TheBecome_Root_Secondary command 320 also utilizes network diameter as anargument and modifies the hello time, maximum age and forward delayparameters stored at the respective memory 240 of switch 121 inaccordance with Table 1, as reflected by block 326.

[0049] It should be understood that other similar commands may beentered to designate third and fourth in line root switches.

[0050] Turning next to FIG. 3C, an Enable_Uplinkfast command 330 ispreferably entered at each access switch 114-116 within the network 100,as shown at block 332. This command 330, at block 334, preferablyincreases the respective switch ID of each of the access switches114-116 to preclude any of the access switches 114-116 from becoming theroot. Specifically, the Enable_Uplinkfast command 330 preferablyincreases the settable portion of the switch ID stored at respectivememory 240 from the IEEE default value of “32768” to “49152”. At block336, the Enable_Uplinkfast command 330 also significantly increases thepath costs of all the ports of the respective access switch. Forexample, the path costs stored at memory 240 for each port arepreferably increased from the IEEE default value of “10” (for 100 Mbpsports) to “3000”. The Enable_Uplinkfast command 330, at block 338, alsoconfigures rapid reconfiguration entity 234 to rapidly transition ablocked port to the forwarding state and to generate and transmit dummymulticast messages, as described below.

[0051] Upon start-up, the ports 230 of each switch, such as switch 214,are initially placed in the blocked state and spanning tree engine 236begins formulating and transmitting bridge protocol data unit (BPDU)frames from each port 230. Presumably, switch 214 has yet to receive anyBPDUs; switch 214 thus assumes that it is the root and transmits BPDUsfrom every port 230 containing its switch ID as the root ID and a rootpath cost of zero. Spanning tree engine 236 obtains the switch ID and amaximum age value for loading in BPDUs from entity 234 and copies theinformation contained in transmitted BPDUs into its associated spanningtree table 242.

[0052] As BPDUs are received at the ports 230 of switch 214, they arepassed to the spanning tree engine 236 and processed. Specifically,engine 236 compares information contained in the received BPDUs with theinformation stored in table 242. If the information from the receivedBPDU is better (e.g., a presumed root with a lower ID) than the storedinformation, engine 236 enters the received information into table 242.Engine 236 also sets the corresponding timer according to the maximumage value in the received BPDU and stops forwarding BPDUs through thisport 230. Execution of the spanning tree algorithm will converge withthe election of a single root by all of the switches in the network 100.

[0053] As discussed above, location of the root may affect the averagedistance (i.e., the average number of hops) a message travels and/orcause undesirable partitioning of backbone switches. The conventionalspanning tree algorithm, however, simply selects the switch having thelowest ID to become the root, regardless of where in the network 100this switch is located. As a result, an access switch is often electedto be the root, causing messages to travel a much higher averagedistance than if a backbone switch had been elected the root for manynetwork configurations, such as partially meshed distributed networks.

[0054] Since the numeric switch ID of the first backbone switch 120 islowered to a value below that of all other switches in the network 100(through the Become_Root_Primary command 300), first backbone switch 120is elected the root switch. Thus, all ports of the first backbone switch120 are placed in the forwarding state. In addition, since the maximumage and forwarding delay parameters are copied by each switch from BPDUsoriginating from the root through operation of the spanning treealgorithm, the values selected from the Become_Root_Primary command 300(see Table 1) are effectively propagated to all switches 114-116 and121-125 within network 100 (FIG. 1).

[0055] For all LANs coupled to both an access switch and a backboneswitch, operation of the above commands 300, 320, 330 also results inthe respective backbone switch becoming the designated switch, ratherthan the access switch. In particular, by significantly increasing thepath costs (e.g., from “10”, to “3000”) for all ports of the accessswitches 114-116, the Enable_Uplinkfast command 330 essentially preventsaccess switches from becoming designated switches on shared media ortrunk links when the second switch is a backbone switch. With referenceto LAN 109 which is coupled to both access switch 116 and backboneswitch 125, for example, only one switch port connected to LAN 109 willbe deemed the designated switch port by the spanning tree algorithm.Determination of the designated switch, moreover, depends in part on thepath costs at the relevant ports of the two switches coupled to LAN 109.Since the path costs at access switch 116 have been increased to “3000”,whereas the path costs at backbone switch 125 remain at the defaultvalue of “100” for ports operating at a data rate of 10 Mb/s or “10” forports operating at 100 Mb/s, the port at backbone switch 125 coupled toLAN 109 will be deemed the designated switch port and placed in theforwarding state. Furthermore, unless the port at access switch 116coupled to LAN 109 represents the root port for switch 116, it will beplaced in the blocked state.

[0056] Significantly, for each access switch 114-116, only one port(local or trunk) that represents a path from the access switch to theroot (i.e., provides connectivity to the root through links, sharedmedia, switches, etc.) will be forwarding. All other ports (local ortrunk) that represent paths from the access switch to the root will beblocked. In other words, only one port at each access switch 114-116that provides connectivity to the root will be forwarding.

[0057] As messages (i.e., data frames) are subsequently received at theports 230, they are passed to forwarding entity 238, assuming therespective port 230 is in the forwarding state. The forwarding entity238 first examines the destination address of the message and performs alook up function at the filtering database 244. Assuming an entry isfound and a corresponding destination port (e.g., port number four) isidentified, the message is switched out onto this port, assuming thisport is in the forwarding state. If no entry is found in the filteringdatabase 244, the message is flooded out all ports 230 in the forwardingstate, except the port 230 on which the message was received. Theforwarding entity 238 next examines the source address of the messageand performs another look-up at filtering database 244. If no entry isfound for the source address, a new entry is formed and the source port(e.g., port number eight) on which the message was received is enteredas the destination port in the corresponding port column. In addition, atimer associated with this entry is set to the aging time with whichforwarding entity 238 is configured (e.g., five minutes).

[0058] Should a change occur in the network 100, such as a failuredisabling the link coupled to a root port of an access switch, theaffected access switch will be able to rapidly reconfigure the network100 without the significant interruption or message flooding experiencedthrough conventional operation of the spanning tree algorithm. Withreference to switch 114, for example, assume that port number three,which is coupled to backbone switch 122, is the root port for switch 114and thus in the forwarding state. Pursuant to the Enable_Uplinkfastcommand 330, port numbers two and four, which also connect switch 114 tothe backbone switches (i.e., backbone switches 122 and 124), are in theblocked state. If the link 128 coupled to the port number three atswitch 114 fails, either one of these two other ports (i.e., portnumbers two or four) will immediately transition to the forwarding stateand begin receiving and sending messages.

[0059] First, the time taken to detect such a change is substantiallyreduced by operation of the novel commands of the present invention. Inparticular, as shown in FIG. 3A and as provided in Table 1, theBecome_Root_Primary command 300 at block 314 causes the maximum agevalue to be significantly lowered from the default value of twentyseconds. For example, if the network diameter is five, the maximum ageis reduced to ten seconds. As described above, this value is loaded intoBPDUs originating at the root (i.e., backbone switch 120), causing it tobe propagated to and stored by each switch at its respectivereconfiguration memory 240. Accordingly, access switches 114-116 andbackbone switches 120-125 detect failures much sooner since thecorresponding BPDU information times out sooner. To prevent BPDUinformation from being inadvertently discarded due to the reduction inmaximum age values, the hello time is also reduced to one second. Thisincreases the frequency with which the root transmits BPDUs.

[0060] Rather than wait for the corresponding BPDU information to timeout, a link failure may alternatively be detected by a link integritytest which operates at the physical layer of the protocol stack. Thelink integrity test typically exchanges test messages across therespective link 128 at a relatively high rate (e.g., every tenmilliseconds). Thus, the link integrity test is able to detect a failuremuch sooner than the spanning tree algorithm, which simply waits for therespective BPDU information to time out and be discarded.

[0061]FIG. 3D is a flow diagram of a rapid reconfiguration process 340following a link failure according to the present invention. In responseto the detection of a failure at port number three (the root port),indicated at block 342, rapid reconfiguration entity 234 at switch 214selects a backup port to become the new root port, as shown at box 344.Rapid reconfiguration entity 234 may use the spanning tree algorithm toselect the next root port. That is, the blocked trunk port 119 (e.g.,port number four) representing the next lowest root path cost (after thenow failed root port) may be selected as the new forwarding port byentity 234. Self-looping ports, such as port numbers five and six atswitch 114 are not considered possible back-up ports, even though atleast one of these ports will be in the blocked state, since these portswill not provide connectivity to the root.

[0062] It should be understood that other methods may be used to selectthe new root port. For example, the blocked trunk port having the lowestport number (e.g., port number two) may be selected.

[0063] Rapid reconfiguration entity 234, at block 346, then directs thespanning tree state machine engine 236 to immediately transition theselected back-up port (e.g., port number four) to the forwarding state.That is, the spanning tree engine 236 does not transition the selectedback-up port between the listening or learning states. Instead, theselected back-up port transitions directly to the forwarding state underthe direction of rapid reconfiguration entity 234, and switch 114immediately begins transmitting and receiving messages through thisnewly activated trunk port 119 (e.g., port number four).

[0064] Since none of the other trunk ports 119 which connect switch 114to, the backbone switches (i.e. to the root) are in the forwarding stateno circuitous path or loop will result from the transition directly tothe forwarding state. That is, by preventing access switches 114-116from becoming the root and configuring the access switches 114-116 toblock all but one of their ports 118, 119 to the backbone switches(i.e., all but one path to the root), through the operation of the abovecommands, a back-up port may be safely transitioned directly to theforwarding state. Ports 118, 119 that provide connectivity downstreamrelative to the root (i.e., to the leaves of the spanning tree) need notbe blocked, since they cannot cause loops.

[0065] Nonetheless, the transition at access switch 114 from initialroot port number three to back-up root port number four may causeentities to appear to “move” relative to other devices. Thus, theidentity of the new root port (e.g., port number four) must bepropagated to the other devices, such as access switches 115-116 andbackbone switches 120-125 to prevent messages from being lost ormisdirected. Switch 114, via rapid reconfiguration entity 234,preferably informs the other devices of its new forwarding port bytransmitting dummy multicast packets through the new port, as indicatedat block 348. FIG. 4 is a highly schematic illustration of a dummymulticast message 400 preferably utilized by rapid reconfigurationentity 234. Dummy multicast message 400 includes a destination address(DA) field 410 and a source address (SA) field 412 that may becompatible with the Media Access Control (MAC) layer of the protocolstack. In the preferred embodiment, dummy multicast message 400 complieswith the IEEE 802.3 standard and includes conventional logical linkcontrol (LLC) and SubNetwork Access Protocol (SNAP) encapsulation, whichare well known to those skilled in the art. Rapid reconfiguration entity234 preferably loads DA field 410 with a multicast address that causesmessage 400 to be received by all devices (e.g., access switches 114-116and backbone switches 120-125) within the network 100.

[0066] Next, rapid reconfiguration entity 234 (FIG. 2) loads SA field412 with the address of an entity directly coupled to switch 114. Forexample, in a first multicast message 400, entity 234 loads the addressof server 112 a into the source address field 412. Although multicastmessage 400 may further include a data field 414, its contents arepreferably not asserted by entity 234. Multicast message 400, having theaddress of server 112 a loaded in SA field 412, is then forwardedthrough the new root port (i.e., port number four) of switch 114.Multicast message 400 is first received at backbone switch 124 whichexamines its contents. In particular, backbone switch 124 notes that themessage is from server 112 a, but that it was received on a differentsource port than switch 124 previously associated with server 112 a.That is, prior to the failure at port number three at access switch 114,backbone switch 124 likely associated all entities directly coupled toswitch 114 (including server 112 a) with its port coupled to backboneswitch 121, which, in turn, accessed such entities via backbone switches120 and 122.

[0067] Since the multicast message 400 having the source address ofserver 112 a was received by backbone switch 124 at a new source port,this new location information is entered by backbone switch 124 into itsfiltering database, replacing the previous information that was storedtherein. Thereafter, if backbone switch 124 receives a message intendedfor server 112 a, it will use this new destination port and the messagewill be received at port number four of switch 114, which is now capableof receiving and forwarding messages. Accordingly, the dummy multicastmessage 400 effectively apprises backbone switch 124 of the change inforwarding ports that occurred at access switch 114. Backbone switch 124distributes the multicast message 400 through all of its forwardingports so that other devices, as necessary, may learn of the newforwarding port (i.e., port number four) at access switch 114.

[0068] Rapid reconfiguration entity 234 similarly generates one or moreseparate multicast messages 400 for each remaining entity directlycoupled to access switch 114 (i.e., entities or end stations on LANs102-104). In each message 400, entity 234 loads SA field 414 with theaddress of the corresponding entity. Each of these messages aresimilarly forwarded by access switch 114 through the new forwarding port(i.e., port number four). In addition, entity 234 generates andtransmits one or more multicast messages 400 for entities reachablethrough switch 114 on ports other than its root port. For example, ifport number seven at switch 114 were coupled to another access switch,entity 234 would transmit multicast messages 400 carrying the sourceaddress of entities directly coupled to this additional access switch aswell. Thus, at least one multicast message 400 is generated andtransmitted for each entity directly coupled to or downstream of accessswitch 114 (relative to the root). As set forth above, upon receipt ofthese multicast messages 400, the other devices within network 100update their corresponding filtering databases with the new destinationports, as necessary.

[0069] In the preferred embodiment, entity 234 is configured to limitthe rate at which dummy multicast messages 400 are transmitted tofifteen messages per one hundred milliseconds or less. This limit onmessage throughput prevents any access switch from consuming asignificant portion of the communications resources of the network 100with the transmission of dummy multicast messages 400.

[0070] The present invention also provides for rapid reconfigurationwhen a new link (or switch), representing a better path the root for agiven switch, is added or recovered. In particular, FIG. 3E is a flowdiagram of a rapid configuration process 350 corresponding to a linkrecovery or addition in accordance with the present invention. Forexample, assume that backbone switch 121 (FIG. 1) is the root and thatport number three at access switch 114 is the root port, since the link128 coupled to port number four (which represents a better path cost toroot 121) is failed. If the link 128 coupled to port number four issubsequently recovered, switch 114 will detect the change, as indicatedat box 352, through the receipt of BPDUs on this port. The recovery oflink 128 may also be detected by the link integrity test, as describedabove.

[0071] As shown at block 354, rapid reconfiguration entity 234 monitorsthe receipt of messages (e.g., BPDUs) at port number four for a periodof time (e.g., thirty seconds) to ensure that the corresponding port atthe upstream switch (e.g., backbone switch 124) has transitioned to theforwarding state. In particular, since backbone switch 124 stilltransitions its ports between the listening and learning intermediarystates, entity 234 at switch 114 preferably waits a period equal totwice the forwarding delay before starting the transition process. Ifentity 234 were to immediately transition to a new port, a loss ofconnectivity might result as the corresponding upstream port may not beforwarding. While port number four is being monitored, moreover, portnumber three remains in the forwarding state and messages may continueto be forwarded and received during this time. Next, after allowing allmessages queued on the current root port (i.e., port number three) to besent, rapid reconfiguration entity 234 transitions the current root portto the blocked state, as indicated at block 356. At or about the sameinstant, rapid reconfiguration entity 234, at box 358, directs thespanning tree state machine engine 236 to transition the recovered port(i.e., port number four) directly from the blocked state to theforwarding state without transition through the listening or learningintermediary states. Following this transition, switch 114 mayimmediately begin transmitting and receiving frames, including datamessages, from this recovered port.

[0072] Next, rapid configuration entity 234 informs the network 100(FIG. 1) of the new forwarding port, as reflected by block 360. Inparticular, as described above, entity 234 generates and transmits dummymulticast messages 400 for all entities directly coupled to switch 114or downstream thereof (relative to the root) through the new forwardingport (i.e., port number four). The transition to a recovered port, whichrepresents a better path to the root, is thus accomplished without theloss of connectivity that otherwise occurs under operation of theconventional spanning tree algorithm. Additionally, the disadvantagescaused by the flooding of messages is also avoided.

[0073] Since the Enable_Uplinkfast command 330 is preferably not enteredat the backbone switches 120-125, these switches do not designateblocked ports as potential back-up ports that may be immediatelytransitioned to the forwarding state. Nonetheless, through theBecome_Root_Primary and Become_Root_Secondary commands 300, 320, thetime required to reconfigure the network following a change at abackbone switch is also reduced. First, as described above, these twocommands 300, 320 significantly reduce the maximum age value utilized byall switches in the network 100, thereby reducing the time it takes todetect a change. Additionally, the forward delay time is lowered asreflected in Table 1. By reducing the time spent in the listening andlearning states, the speed at which network 100 is reconfiguredfollowing a change is substantially improved.

[0074] It should be understood that switch 114, in addition togenerating and transmitting dummy multicast messages 400 following thedetection of a change, may also transmit TCN-PDUs toward the root. Asdescribed above, by transmitting TCN-PDUs, switch 114 will causedevices, such as switches 114-116 and 120-125, to shorten the age outtime associated with their filtering databases 244. By quicklydiscarding the contents of their filtering databases 244, thepossibility of data messages being misdirected or lost is significantlyreduced. Furthermore, the transmission of dummy multicast messages 400,as described above, quickly informs devices of newly activated ports,thereby reducing the flooding that otherwise occurs when the contents offiltering databases 244 are discarded.

[0075] The above commands 300, 320, 330 also reduce the disruptions thatmay occur upon failure of the root. A failure at the root (e.g.,backbone switch 120) will suspend the transmission of BPDUs within thenetwork 100. The BPDU information stored at the remaining devices, suchas access switches 114-116 and backbone switches 121-123, will then timeout, based on the particular maximum age value obtained from Table 1. Inresponse, these devices will begin recalculating the spanning tree forthe network 100. Since the Become_Root_Secondary 320 command was enteredat backbone switch 121, its corresponding numeric identifier wassignificantly lowered, causing backbone switch 121 to become the newroot. The Become_Root_Secondary command 320 thus ensures that, upon afailure of the primary root (i.e., backbone switch 120), the next switchelected to be the root will also be a backbone switch. By carefullyselecting the second backbone switch 121, the average distance traveledby messages within the network 100 may be optimized and partitioning ofbackbone switches avoided. BPDUs originating from the new root (e.g.,backbone switch 121), moreover, will contain the improved parameters setforth in Table 1, by virtue of the Become_Root_Secondary command 320.Thus, switches 114-116 and 120-125 will be able to rapidly reconfigurethe network 100 in the face of changes following the election of the newroot (e.g., the backbone switch 121).

[0076] Virtual Local Area Networks

[0077] A computer network, such as network 100 (FIG. 1), may also besegregated into a series of network groups. For example, U.S. Pat. No.5,394,402, issued on Feb. 28, 1995 to Floyd E. Ross (the “'402 patent”),which is hereby incorporated by referenced in its entirety, discloses anarrangement that is capable of associating any port of a switch with anyparticular segregated network group. Specifically, according to the '402patent, any number of physical ports of a particular switch may beassociated with any number of groups within the switch by using avirtual local area network (VLAN) arrangement that virtually associatesthe port with a particular VLAN designation. More specifically, Rossdiscloses a switch or hub for a segmented virtual local area networkwith shared media access that associates VLAN designations with at leastone local port and further associates those VLAN designations withmessages transmitted from any of the ports to which the VLAN designationhas been assigned.

[0078] The VLAN designation for each local port is stored in a memoryportion of the switch such that every time a message is received by theswitch on a local port the VLAN designation of that port is associatedwith the message. Association is accomplished by a flow processingelement which looks up the VLAN designation in a memory based on thelocal port where the message originated. In addition to the '402 patent,an IEEE standards committee is preparing a standard for Virtual BridgedLocal Area Networks. See IEEE Standard 802.1Q (draft).

[0079] In many cases, it may be desirable to interconnect a plurality ofthese switches in order to extend the VLAN associations of ports in thenetwork. Ross, in fact, states that an objective of his VLAN arrangementis to allow all ports and entities of the network having the same VLANdesignation to interchange messages by associating a VLAN designationwith each message. Thus, those entities having the same VLAN designationfunction as if they are all part of the same LAN. Message exchangesbetween parts of the network having different VLAN designations arespecifically prevented in order to preserve the boundaries of each VLANsegment. For convenience, each VLAN designation is often associated witha different color, such as red, blue, green, etc.

[0080] A separate spanning tree or active topology may be defined foreach VLAN designation defined within the network. See Cisco IOS VLANServices document. That is, a first spanning tree may be associated withthe red VLAN designation and a second spanning tree associated with theblue VLAN designation. Thus, a given port may be in the forwarding statefor a first VLAN designation (e.g., red), but blocked for second VLANdesignation (e.g., blue).

[0081] In a preferred embodiment, the Become_Root_Primary andBecome_Root_Secondary commands 300, 320 are associated with one or moreVLAN designations. That is, for the red VLAN designation, theBecome_Root_Primary command 300 may be entered at backbone switch 120,whereas, for the blue VLAN designation, it may be entered at backboneswitch 121. Thus, the root for the spanning tree associated with the redVLAN designation will be at backbone switch 120 and the root for theblue VLAN designation will be at backbone switch 121. By establishing adifferent root for the various VLAN designations, improved loadbalancing may be achieved on the network. For example, if the red andblue VLAN designations each generate high message traffic, then, bydesignating different roots for these VLAN designations, the paths orlinks followed by messages with the red VLAN designation will bedifferent than the paths or links followed by messages with the blueVLAN designation. Other low-traffic generating VLANs (e.g., yellow andgreen) may be divided between the two high traffic VLANs. That is, thered and yellow VLAN designations may share backbone switch 120 as theirroot, while the blue and green VLAN designations may share backboneswitch 121 as their root. This provides an even greater measure of loadbalancing within the network 100.

[0082] In particular, the Become_Root_Primary and Become_Root_Secondarycommands 330, 320 may include a VLAN list as a second argument inaddition to the network diameter. For example, at first backbone switch120, the Become_Root_Primary command 300 may be entered with a networkdiameter of four and a VLAN list identifying the red, yellow, orange andviolet VLAN designations as its arguments. At second backbone switch121, the Become_Root_Primary command 300 may be entered with a networkdiameter of four and a VLAN list containing the blue, green and magentaVLAN designations as its arguments. In response, switch 120 will becomethe root for the red, yellow, orange and violet VLAN designations, whileswitch 121 will become the root for the blue, green and magenta VLANdesignations.

[0083] Further load balancing may be achieved by having access switchesselect root ports on a per-VLAN basis. This may be accomplished bymodifying the path costs for each trunk port of the access switches on aper-VLAN basis. In particular, the Enable_Uplinkfast command may bemodified to adjust the path costs for each trunk port at the respectiveswitch on a per-VLAN basis by adding a port number, VLAN designation andrespective path cost string as an argument to the Enable_Uplinkfastcommand. For example, the Enable_Uplinkfast command 330 may be enteredat access switch 114 with the following argument string: port numbertwo, red, 3100; port number two, blue, 3200; port number three, red,3200; port number three, blue, 3100; port number four, red, 3300; portnumber four, blue, 3300. Since port number two has the lowest path costfor the red VLAN designation, this port will generally become the rootport for the red VLAN. For the blue VLAN designation, port number threehas the lowest path cost and thus will become the root port for the blueVLAN. Accordingly, all messages associated with the red VLAN designationwill be forwarded and received at port number two and all messagesassociated with the blue VLAN designation will be forwarded and receivedat port number three. By dividing the message streams among differenttrunk ports 119, improved load balancing may be achieved.

[0084] Referring again to FIG. 4, in formulating dummy multicastmessages following the detection of a change, access switches 114-116may append a VLAN tag 416 to message 400. The VLAN tag 416 is preferablyloaded with the VLAN designation corresponding to the entity whoseaddress is loaded into SA field 412. Although VLAN tag 416 may beappended to the message 400, as shown, it is preferably inserted afterthe source address field 412. It will be understood to those skilled inthe art that tag 416 may be inserted at other locations.

[0085] As switches typically support the creation of up to “1024”different VLAN designations, substantial memory may be needed to store adifferent path cost for each VLAN per port. In the preferred embodiment,path costs are permitted to take only one of two possible values, one ofwhich is a default value. Accordingly, a “128” byte vector can beutilized per trunk port to represent the path costs for each VLANdesignation. That is, each bit of the vector represents one VLANdesignation. By parsing each bit, rapid reconfiguration entity 234 candetermine the path cost for each VLAN designation. More specifically, ifthe bit is asserted, then the path cost for the corresponding VLANdesignation will be set to the new value (e.g., “3100”). If a bit is notasserted, then the path cost for the corresponding VLAN designation willremain at the default value (e.g., “3000”).

[0086] The foregoing description has been directed to specificembodiments of this invention. It will be apparent, however, that othervariations and modifications may be made to the described embodiments,with the attainment of some or all of their advantages. Therefore, it isthe object of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

What is claimed is:
 1. A method for rapidly re-configuring a computernetwork having a plurality of access devices and backbone devices eachhaving a unique numeric identifier with a settable portion, each accessdevice including a plurality of ports for coupling the respective accessdevice to at least one of the access and backbone devices and each porthaving an associated path cost, the method comprising the steps of:decreasing a settable portion of the numeric identifier at a firstbackbone device below a first default value; for at least one accessdevice, increasing a settable portion of the respective numericidentifier above the first default value and all path costs of therespective ports above a second default value; executing a spanning treealgorithm (i) to elect the first backbone device as a root in responseto the step of decreasing and (ii) to place a single port at the atleast one access device, providing upstream connectivity to the root, ina forwarding state while further placing all remaining ports of the atleast one access device, providing upstream connectivity to the root, ina blocked state in response to the step of increasing, and in responseto a failure at the single forwarding port, providing upstreamconnectivity to the root, of the at least one access device,transitioning a blocked port, providing upstream connectivity to theroot, from the blocked state directly to the forwarding state.
 2. Themethod of claim 1 further comprising the step of designating the blockedports of the at least one access device, providing upstream connectivityto the root, as backup ports.